Monday, August 9, 2010

CentOS / Red Hat Configure an NTP Client And Server

Install ntp

The ntp package contains utilities and daemons that will synchronize
your computer's time to Coordinated Universal Time (UTC) via the NTP
protocol and NTP servers. The ntp packageincludes ntpdate (a program for
retrieving the date and time from remote machines via a network) and
ntpd (a daemon which continuously adjusts system time). Install the ntp
package:# yum install ntp

How do I configure an NTP Client?

Simply open /etc/ntp.conf file, enter:# vi /etc/ntp.conf
Make sure the following line exists:server ntp.server.com
Where,

ntp.server.com : the hostname or IP address of the site NTP server. If your ntp server located at 192.168.1.5, enter server 192.168.1.5. You can also use public ntp server located at ntp.org.

You can also run ntpd using cron:# echo '30 * * * * root /usr/sbin/ntpd -q -u ntp:ntp' > /etc/cron.d/ntpd
The above instructs crond to run ntpd and after setting the clock just
exit, and the -u option instructs it to run as the ntp user.

Configure an NTP Server

If you have lots of server and desktop system, configure your own NTP
server. Your NTP server contacts a central NTP server,provided by your
ISP or a public time
server located at ntp.org, to obtain accurate time data. The server then
allows other machines on your network to request the time data. Our
sample setup:

First, install and enable ntpd on 192.168.1.5:# yum install ntp
# chkconfig ntpd on
Now open /etc/ntp.conf:# vi /etc/ntp.conf
Make sure the following line exits:restrict default ignore
Above will deny all access to any machine, server or client. However,
you need to specifically authorized policy settings. Set it as follows:

Replace 202.54.1.5 and mask with actual remote ISP or ntp.org NTP server IP. Save and close the file.

Configure NTP clients to access your NTP Server

Now, you need to allow legitimate NTP clients to access the Server.
For example, allow 192.168.1.0/24 network to synchronize to this server
located at 192.168.1.5. Open /etc/ntp.conf and add policy as follows: